Resedimentation of the 1.8 ka Taupo ignimbrite in the Mohaka and Ngaruroro river catchments, Hawke's Bay, New Zealand

Abstract

Deposition of the 1.8 ka Taupo ignimbrite in the Hawke's Bay buried parts of the Ngaruroro and Mohaka River catchments beneath up to 40 m of loose pyroclastic debris. Re‐establishment of the two river systems led to the remobilisation of the loose debris and followed similar patterns in both catchments. An initial period of laharic remobilisation and formation of lahar‐deposit‐dammed lakes was followed by two phases of fluvial remobilisation. During the first of these, streams were shallow and ephemeral to perennial, with sediment‐laden flash‐floods eroding headwards through valley‐ponded primary pyroclastic and laharic deposits. With gradual re‐establishment of vegetation, the sediment:water ratio decreased, and braided rivers with deeper, more stable channels became dominant. Although the general pattern of resedimentation was similar for both river systems, field evidence shows that the volumes of remobilised material, and the likely durations of resedimentation in the different catchments, differed. These variations reflect: (1) the different volumes of initial ignimbrite in the trunk valleys; (2) the different percentage of catchment areas covered by ignimbrite debris; and (3) the different river gradients and valley shapes. However, extremely high sediment yields in the Hawke's Bay region are estimated to have persisted for at least 4–17 yr after the eruption.